1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process, such as a laser beam printer, a copying machine, or a facsimile.
2. Description of the Related Art
A method for forming a color image with an electrophotographic image forming apparatus is known by which a color image is formed by overlapping images of a plurality of colors by repeating a process of transferring a toner image formed on a photosensitive drum through charging, exposure, and development onto a recording medium a plurality of times. It is known that a phenomenon occurs in such a color-image forming apparatus in which white gaps, which should not be present, are generated between adjacent images of different colors occurs. This phenomenon is hereinafter referred to as white gap. This phenomenon occurs when a visualized image thinner than an electrostatic latent image formed on the photosensitive drum is formed when an electrostatic latent image, such as a latent image of an image edge, which is formed on a photosensitive drum due to an abrupt change in potential, is developed by a developing unit. For example, in the case of an image in which a cyan belt and a black belt are next to each other, a thin cyan visualized image and a thin black visualized image are formed, although the color belts should be next to each other. This causes a gap between the color belts in a final image on a recording medium, resulting in degradation in image quality. FIG. 19 is a diagram illustrating a state in which white gaps are generated, showing the state of an electric field between a photosensitive drum and a developing roller. The thinning of a visualized image at a visualizing portion, which causes white gaps, occurs because an electrostatic latent image formed on an electrostatic portion of a photosensitive drum wraps the electric field at the edge.
Japanese Patent Laid-Open No. 2003-312050 discloses an image forming apparatus in which a minute laser beam is emitted to a non-toner-image formation section on a print area to the extent that toner does not adheres excessively, thereby attaining an appropriate potential of the photosensitive drum. Such weak exposure of a non-toner-image formation section (a white portion) is referred to as background exposure.
The background exposure is performed also not for preventing the above white gaps. Japanese Patent Laid-Open No. 2012-189886 discloses an in-line color-image forming apparatus that performs background exposure to attain an appropriate potential of a non-toner-image formation section of a photosensitive member when using a common charging bias power supply or a common developing bias power supply.
An example of a method for performing background exposure (weak exposure) is a method of changing the duty ratio of pulse waves, called pulse width modulation (PWM). This is a method of causing an exposure device to emit light to one pixel in synchronization with an imaging clock only a period of background exposure (a pulse width) corresponding to a minute exposure amount to the extent that no toner adheres, which is shorter than a normal exposure period (a pulse width) for adhesion of toner.
However, the background exposure based on PWM can generate image defect depending on the allocation of an emission period for normal exposure for toner adhesion and an emission period for background exposure for preventing toner adhesion.
In other words, simply placing the emission period for background exposure in an emission period not for normal exposure, which corresponds to a white portion of the image, can cause exposure light during the emission period for background exposure to interfere with exposure light during light emission for normal exposure. The interference here is a state in which an area of the photosensitive member corresponding to the emission period for normal exposure and an area corresponding to the emission period for background exposure are excessively close to each other.
Such interference decreases the potential of an area of the photosensitive member between the area corresponding to the emission period for normal exposure and the area corresponding to the emission period for background exposure. Thus, the potential of the sandwiched area decreases to an exposure level at which toner adheres, so that an area in which toner adheres finally unintentionally increases in size. This can cause image defect.